Pavement design - good, cheap or quick: Can I have 80% of all of the above?

[ACtrafficengr]

This isn't really a road question, per se. It's for a multi-use path. To build it the way we want (4"/100mm of HMA on 6"/150mm subbase) would cost $4.5 million. Available funding is less than 1/3 of that.

So, one of the other engineers in the office suggested a surface treatment instead of hot mix. We don't really have any experience with surface treatments (although they'd probably get a higher bang/$ ratio on our low volume roads, but that's getting off topic). It would reduce the cost significantly, and should suffice for a surface that will be lightly loaded once the construction traffic is off it. Typical axle loads will be under 4 kip/1800kg (a few maintenance patrols per week with a 1 ton pickup).

The problem is, bicyclists (including myself) hate riding on chip seals. The increased vibration and rolling resistance is highly annoying. I'd imagine they are even worse for skaters.

So, I poked around the interwebs and I found this report:

Preliminary Results: Measurement of Macrotexture on Surface Treatments and Survey of Bicyclist Ride Quality on Mon-198 and SLO-1 Test Sections​

which states that 80% of cyclists surveyed found a mean profile depth (MPD) of under 2 mm acceptable. It also provides MPDs of a number of surface treatments, and it several are under that 2 mm mark. Options include microsurfacing, cape seal, double chip seal, among others.

According to this report,

Best Practices for Bicycle Trail Pavement Construction and Maintenance in Illinois​

6" of subbase, asphalt emulsion primer, and a surface treatment should provide a suitable cross section. It may be more than sufficient where the subbgrade is firmer, but this report,

Guide to Bikeway Pavement Design, Construction and Maintenance for South Australia​

recommends thicker subbases - as high as 360 mm. I've never been to South Australia, but our climate is humid continental - similar to Illinois.

So, my questions are:
[ol 1]
[li]For a pavement which will be lightly loaded after construction, would the Illinois section be sufficient, or should we use S.A.'s deeper subbases? [/li]
[li]Would a performance specification work better than our normal method and materials specification? In other words, should we make the contractors bid on a surface treatment of our choosing, or should we specify the MPD and traffic loading, and see what contractors can come up with? We may lose some control with the latter, but say we choose a cape seal. We would exclude all the contractors that aren't equipped for that. We might be excluding a contractor that could give us that higher bang/$ ratio.[/li]
[/ol]

Thanks for any insights you can provide.

 

[DDBPE]

A couple questions.
1. By lightly loaded, is this light vehicle use for maintenance purposes or for recreational use (pedestrians and cyclist)?
2. What type of cyclist, road or off road. Road cyclist do not like chip seals, at least initially. Typically on roads, the cars will help compact the loose material and also help clean the road of any material not embedded in the liquid. If this is a trail, this will take a long time or you may have to clean up loose material after construction.
3. What is the subgrade material is it suitable. A CBR value would be good information.
4. Any other requirements for use or required loading?

I have bid on and constructed several asphalt trails, parking lots, roads, etc. Typically for trails we see 4-6" of dense graded aggregate with only 1-1/2" of asphalt on a decent subgrade. I have built light use parking lots with 8" of stone and 1-1/2"- 2-1/2" of asphalt.

It just seems like overkill for 4" of asphalt if the use is strictly recreational.

If you do a performance specification, it would be difficult to reasonably compare different treatments. If you went that route, it would be best to have them design and bid on one treatment (asphalt or surface treatment but not both). Then you could compare apples to apples. We do these type analysis for bidding design build projects when needing to maximize pavement life and cost. Different pavement sections can result in the same strength but have significantly different cost. Also, how would you consider any maintenance cost in evaluating the different treatments.

 

[ACtrafficengr]

Thanks for answering.

I'm assuming foot and bike traffic would have neglegible effect on the surface, so maintenance trucks will govern design.

I don't like riding on chip seals, so that's why I read that first report. There do seem to be some viable alternatives.

The subgrade is a former railbed. We don't have a penetrometer or any other good way to get CBRs. Some areas are in cuts and the ditches were not maintained since the RR discontinued service in 2003. I'm assuming they might be a bit mushy.

We've had problems in this area with thin HMA pavements. Either they rut in the summer, even though the heaviest traffic is pickup trucks, or crack in the winter.

The railroad was built on a deeded property, not an easement, so we (probably) aren't affected by the recent SCOTUS ruling.